9700 mile sounds good. I have had a few that only had 7000+ on them but were locked up due to poor storage conditions and required rebuild. None of these transmissions showed any damage and have been working well without worry. Have you had a conversation with the PO that would lead you to believe that he has damaged the transmission? A constant false neutral going int 3rd gear? clutch issues? The rod that runs through the clutch isn't problematic as a rule but there has been a few on this forum that have found this rod to be broken( cracked and broken into 2 pieces) I have only found this once , and that was on a 500 that had been seriously beat. You can pull it out , inspect it and slide it back in without disturbing anything, one end is rounded, so if you have 2 cut ends it's broken and needs to be replaced. The other thing that is common , especially on beat machines is where the clutch cable goes into the case , the lever itself can be found to be cracked or damaged or most of the time if I find trouble with the clutch, it is that somebody dropped the ball bearing when reinstalling the case. That whole movement should be cleaned , inspected and greased, it probably still has the factory grease in it from the early 70's that needs freshening. Since it turns over, it would be worth cleaning the carbs and getting a fresh battery, oil and filter,plugs and clean up the points. to get it it running and try out the transmission out just so you know that you're not chasing a ghost that doesn't exist. There shouldn't be much wear in a low mileage transmission, although the forks can be stuck from sitting for years and years in very poor or outdoor storage situations, you will know immediately when you shift it on the centerstand and nothing happens. If you can turn the rear wheel and get through the gears ...that isn't a problem
So I'm thinking that you are only looking at a top end rebuild just to change the O rings. The top end will need to be broken down to change the oil port O rings between the head and the jugs. That can be done after you get it running and try the transmission out if it is bleeding out heavily. Then you should have an idea of what you really have there.... it may just be an ugly paint job and Orings. Of course you can take the whole thing apart, split the cases and put it back together if you are worried about gaskets and seals in the lower end , we all love to see pictures of that stuff whether you need to do it or just want to do it, your choice , we like it either way. Especially if you're on a budget, the exhaust is $350 or more just for that alone, 100 + for gaskets , another !00+ for lower end seals, under 75 for clutch if in good shape, neck beaings shouldn't be worn yet but you may want all balls instead of a simple regrease...whatever , depending on your budget ,it can add up fast.paint alone will set you back 150 for basecoat /clear coat on the low endand a decent seatcover starts around $50, ,then there's brake mc rebuild kit,pads shoes and tires ,really adds up quick. Pop another buck and a half on for electronic ignition instead of points,new coils instead of stock coils, ...it goes on and on, be careful. What are your plans for this machine, it looks like you could bring it back easily.